Organic transistors are widely used as principal semiconductor electronic devices constituting displays and computer devices, and are now produced using inorganic substances, such as polysilicon and amorphous silicon, as semiconductor materials. Disadvantageously, production of thin-film organic transistors using such inorganic substances requires a vacuum process and/or a high-temperature process and causes increased production costs. In addition, the presence of the high-temperature process in the production imposes limitations on substrates that are usable. The substrate to be used in the production has been selected mainly typically from glass substrates. However, the glass substrates, although having good heat resistance, are susceptible to impact, hardly achieve weight reduction, have poor flexibility, and hardly give flexible organic transistors.
As a possible solution to this issue, organic electronic devices using organic semiconductor materials have been studied and developed actively through the years. Advantageously, the organic semiconductor materials can easily form thin films by a simple procedure via a wet process such as printing process or spin coating process and allow the production process to proceed at a lower temperature as compared with the conventional organic transistors using inorganic semiconductor materials. This enables the formation of thin films on plastic substrates, which generally have low heat resistance, and achieves reduction in weight and cost of electronic devices such as displays. In addition, such organic electronic devices are expected to be used in broader, various ways such as uses utilizing the flexibility of the plastic substrates.
When used as the organic semiconductor materials, low-molecular-weight semiconductor materials such as pentacene are known to develop semiconductor device performance at high level. However, most of unsubstituted acene compounds typified by pentacene have poor solubility in common solvents, due to strong intermolecular interaction by the n-conjugated system. The poor solubility impedes the preparation of a high-concentration composition for organic transistor production, but gives a low-concentration composition. The resulting organic semiconductor, when formed by printing process using such a low-concentration composition has smaller crystal grains. Typically disadvantageously, the organic semiconductor is not energized unless a high voltage is applied, and the application of such a high voltage causes the insulating film to be separated.
In addition, Non Patent Literature (NPL) 1 and NPL 2 describe the use of donor-acceptor copolymer compounds having a thiophene skeleton as organic semiconductor materials. The compounds exhibit high mobilities due to strong π-π stacking and resulting high π-electron overlap. Disadvantageously, however, the strong π-π stacking causes the compounds to have high crystallinity and poor solubility in common solvents. The literature describes that the compounds are dissolved by heating using a halogenated solvent typified by 1,2-dichlorobenzene. However, resulting solutions of the compounds, when dissolved typically in 1,2-dichlorobenzene, often gelate at room temperature and are not suitable for thin film formation by a printing process. In addition, such halogenated solvents might cause ecological toxicity and offer working safety hazard.